Claiming amount to be paid without any confirmative documentation

I received a sms from this firm requesting to pay R1294.31 into MTN Trust acc re an outstanding amount on my MTN account ref *** and RA*** as well as their fee inclusive. No documentation. I closed my MTN account ages ago. I went to SAYA's website and by 3 occasions sent them messages from their website asking for documentation backing their claim. No reaction. Got hold of a phone number and sent a sms. No reaction. I am not phoning them, I believe it's their duty to call. They have both my cell number and e-mail address. They have a very bad attitude.